The behavior and physiology of organisms display rhythms of many periodicities. The existence of many endogenous biological rhythms raises the question of their organization. How do they work together to produce an integrated pattern of behavior? The proposed research will focus on the estrous cycle in the albino rat and hamster as an exemplar of other biological multi-oscillator systems. The estrous cycle has already been shown to be influenced by at least two endogenous biological oscillators, one with a circadian periodicity and the other with a periodicity of about 4 days. In her Ph.D. thesis, this investigator tested two models of estrous organization, entrainment and gating, which make different predictions for the length of the estrous cycle in non-24-hour light-dark cycles. The entrainment model was eliminated and the gating model was tentatively supported. However the gating model did not predict the trend toward persistent estrus observed in the non-24-hour groups. The proposed studies carry out experimental tests of more comprehensive hypotheses, internal desynchronization and photoperiodism. The internal desynchronization hypothesis suggests that the estrous cycle lengthens when families of component oscillators become internally desynchronized. The photoperiodic hypothesis suggests that the lengthening of the estrous cycle is a seasonal effect due to simulation of a long day length. The estrous cycles of the rat and hamster will be compared since there are many differences between the two species which may clarify the role of internal desynchronization versus photoperiodism. The proposed research has implications for the control of ovulation in women and as a model for other multi-oscillator biological systems such as birth.